In general, materials such as oxides, resins, glasses and metals have a positive thermal expansion coefficient. However, recently various mixed oxides that exhibit a negative thermal expansion coefficient have been reported by Evans et al.
The U.S. Patents of Evans et al. show a combination of materials having positive and negative thermal expansion coefficients. Examples in which an oxide having a negative thermal expansion coefficient is dispersed in an epoxy resin are shown in the working examples in those patents (see, for example, U.S. Pat. No. 5,322,559, U.S. Pat. No. 5,433,778, U.S. Pat. No. 5,514,360, U.S. Pat. No. 5,919,720, U.S. Pat. No. 6,183,716).
Of these materials having a negative thermal expansion coefficient, materials represented by A3+2(M6+O4)3 have been reported on (J. S. O. Evans et al., “Negative thermal expansion materials”, Physica B, 241–243, 1998, pp. 311–316). In that document (non-patent document), Al, Y and Sc are given as examples of A3+, and W and Mo are given as examples of M6+. Moreover, in that document, Evans et al. report detailed X-ray diffraction analysis results for these materials. According to that report, the mechanism for negative thermal expansion arises from M-O-M bonds. Specifically, M-O bonds expand through heat, but the M—M interatomic distance of M—M bond becomes shorter through vibration of the oxygen. As a result, a negative thermal expansion coefficient is exhibited. This is singular behavior not exhibited by conventional mixed oxides.
According to the above report, for example Sc3+2(W6+O4)3a has a volumetric expansion coefficient αv of −6.5 ppm/K.
Moreover, as other mixed oxides, cordierite type materials such as “Nexcera” (product name, made by Nippon Steel Corporation) are known to exhibit a negative thermal expansion coefficient (see Japanese Unexamined Patent Publication No. 2000-290064, Japanese Unexamined Patent Publication No. 2001-19540).
However, these materials are still of limited types, and are not able to completely answer to the needs of various uses. In particular, in fields such as electronic materials and precision instruments for which low thermal expansion is required to a high degree, the development of low thermal expansion materials each suitable for a particular one of these uses is required.